5.B.5 The Extracellular Metal Oxido-Reductase (EMOR) Family
c-Type cytochromes (c-Cyts) play vital roles in mediating electron transfer (ET) reactions associated with respiration. All c-Cyts possess at least one haem that is covalently bound through amino acid side-chains of the proteins to position and orient the haem moiety and thereby facilitate efficient reactions. The haem moieties are commonly co-ordinated through two thioester bonds to proximal cysteines in the protein, where the signature motif of most c-Cyts is CX2CH (other common motifs include CX3-4CH, CX2CK and A/FX2CH). These motifs with covalently bound haems are the key components used to constitute the haem-containing domains whose functions range from binding of O2 and catalysis to electron transfer and accumulation (Stevens et al., 2004; Rodrigues et al., 2006).
Dissimilatory reduction of metal (e.g. Fe, Mn) (hydr)oxides represents a challenge for microorganisms, as their cell envelopes are impermeable to metal (hydr)oxides that are poorly soluble in water. To overcome this physical barrier, the Gram-negative bacteria Shewanella oneidensis MR-1 and Geobacter sulfurreducens (TC# 5.B.3) have developed electron transfer (ET) strategies that require multihaem c-type cytochromes (c-Cyts) (Shi et al., 2007). In S. oneidensis MR-1, multihaem c-Cyts CymA and MtrA are believed to transfer electrons from the inner membrane quinone/quinol pool through the periplasm to the outer membrane. The type II secretion system translocates decahaem c-Cyts MtrC and OmcA across outer membrane to the surface of bacterial cells where they form a protein complex. The extracellular MtrC and OmcA can directly reduce solid metal (hydr)oxides. Multihaem c-Cyts thus play critical roles in S. oneidensis MR-1- and G. sulfurreducens-mediated dissimilatory reduction of solid metal (hydr)oxides by facilitating ET across the bacterial cell envelope (Shi et al., 2007).
Phototrophic Fe(II)-oxidizing bacteria couple the oxidation of ferrous iron [Fe(II)] to reductive CO2 fixation by using light energy. Rhodopseudomonas palustris TIE-1 has a three-gene operon, designated the pio operon (for phototrophic iron oxidation), that is necessary for phototrophic Fe(II) oxidation. The first gene in the operon, pioA, encodes a c-type cytochrome that is upregulated under Fe(II)-grown conditions. PioA contains a signal sequence and shares homology with MtrA, a decaheme c-type cytochrome from Shewanella oneidensis MR-1 (TC# 5.B.5.1.1). The second gene, pioB, encodes an outer membrane β-barrel protein, a homologue of MtrB from S. oneidensis MR-1. The third gene, pioC, encodes a high potential iron sulfur protein (HiPIP) with a twin-arginine translocation (Tat) signal sequence and is similar to the Fe(II) oxidoreductase (Iro) from Acidithiobacillus ferrooxidans. Like PioA, PioB and PioC appear to be secreted proteins. Deletion of the pio operon results in loss of Fe(II) oxidation activity and growth on Fe(II). Thus, proteins encoded by the pio operon are essential and specific for phototrophic Fe(II) oxidation in R. palustris (Jiao and Newman, 2007).
The autotrophic Sideroxydans lithotrophicus ES-1 can grow by coupling the oxidation of ferrous iron to the reduction of oxygen. Soluble ferrous iron is oxidized at the surface of the cell by an MtoAB porin-cytochrome complex that functions as an electron conduit through the outer membrane. Electrons are then transported to the cytoplasmic membrane where they are used to generate a pmf (for ATP synthesis) and NADH for autotrophic processes such as carbon fixation. As part of the mtoAB gene cluster, S. lithotrophicus also contains the gene mtoD that is proposed to encode a cytochrome c protein. MtoD was chaacteruzed biochemically, biophysicallly, and by x-ray crystallography. MtoD is an 11 kDa monomeric protein containing a single heme that belongs to the class-1 cytochrome c family and had a similar fold to ferricytochrome c552 family. However the MtoD heme is bis-histidine coordinated and is substantially more exposed than the hemes of other family members.
The electron transfer pathway for the Shewanella metal oxidoreductase is:
quinol (cytoplasmic membrane) → CymA (inner membrane, periplasmic side) → MtrA (periplasm) → MtrB (outer membrane) → OmcA/MtrC complex → Fe3+/Mn3+/4+
The multihaem c-type cytochrome quinol:Fe3+ /Mn3 /4+ oxidoreductase, Cym/Mtr (Shi et al., 2007). MtrABC is composed of two decahaem cytochromes (MtrA & B) brought together inside a transmembrane β-porin (MtrC) to transport electrons across the outer membrane to mineral based electron acceptors (White et al. 2012). Conduction through MtrCAB directly to Fe(III) oxides occurs both in vitro in liposomes and in vivo, allowing anaerobic, solid-phase iron respiration (White et al. 2013). MtrC interacts with the surface of MtrAB, extending ∼70 Å from the membrane surface and allowing the terminal hemes to interact with both MtrAB and an extracellular acceptor. MtrA fully extends through the length of MtrB, with ∼30 Å being exposed into the periplasm. MtrCAB can reduce Fe(III) citrate with STC as an electron donor, disclosing a direct interaction between MtrCAB and STC (Edwards et al. 2018). MtrC, but not UndA (a paralog of MtrC of 843 aas; F8UWD6), appears to be the primary reductase of flavins to ensure fast indirect extracellular electron transfer (EET), which plays a crucial role in microbial fuel cell (MFC) electricity generation in Shewanella putrefaciens CN32 (Wu et al. 2018). The dimensions of MtrAB are approximately 105 x 60 x 35 Å and approximately 170 x 60 x 45 Å for MtrCAB. Their shapes suggest that MtrC interacts with the surface of MtrAB, extending approximately 70 Å from the membrane surface and allowing the terminal hemes to interact with both MtrAB and an extracellular acceptor. MtrA fully extends through the length of MtrB, with approximately 30 Å being exposed into the periplasm (Edwards et al. 2018).
Cym/Mtr of Shewanella oneidensis
CymA (quinol dehydrogenase; 187 aas)
(~34% identical to TorY; TC# 5.A.3.4.2)
MtrA (decaheme cytochrome c; 330 aas)
(21% identical to the polyheme membrane associated cytochrome c; TC# 5.B.3.1.1)
MtrB (outer membrane protein precursor with homology to parts of members of the AT family, TC# 1.B.12; 697 aas)
(similar to PioB of the phototrophic iron (Fe2+)
oxidoreductase oxidizing (Pio), CO2 reducing complex of
Rhodopseudomonas palustris; TC# 5.B.5.2.1))
MtrC dodecaheme cytochrome c; 671 aas
OmcA (decaheme cytochrome c; 735 aas;
may be distantly related to cyt c3 of Geobacter sulfurreducens; TC# 5.B.3.1.1)
Four component transenvelope ferrous oxidase, CymA/MtoA/MtoB/MtoD (Beckwith et al. 2015).
Ferrous oxidase of Sideroxydans lithotrophicus
Extracellular respiratory system surface decaheme cytochrome c component MtrF of 639 aas.
MtrF of Shewanella oneidensis
Surface localized decaheme cytochrome c lipoprotein of 759 aas.
Lipoprotein of Shewanella oneidensis